Added number_of_corners attribute.
Added number_of_corners attribute.

amandersillinois/landlab,RondaStrauch/landlab,laijingtao/landlab,RondaStrauch/landlab,csherwood-usgs/landlab,ManuSchmi88/landlab,cmshobe/landlab,Carralex/landlab,laijingtao/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,amandersillinois/landlab,ManuSchmi88/landlab,csherwood-usgs/landlab,cmshobe/landlab,landlab/landlab,Carralex/landlab,cmshobe/landlab,Carralex/landlab,landlab/landlab,landlab/landlab